This invention relates to plastics-encapsulated semiconductor devices, for example power devices, device assemblies and integrated circuits, particularly (but not exclusively) of the surface-mount device (so-called SMD) type. It is concerned with aluminium corrosion in such devices due to moisture.
In semiconductor device manufacture, it is a common technique to use plastics material (synthetic resins) to encapsulate a semiconductor device body. Typically, for power devices, the device body has bond pads of aluminium at a surface thereof. Connection wires are bonded between these bond pads and electrical terminal areas of the device. It is convenient to make the connection wires also of aluminium, having regard to metallurgical compatibility, current-carrying capability and cost. Both the device body and connection wires are located inside the plastics encapsulation, and they are thereby protected from the environment around the device.
However, it is found that plastics-encapsulated semiconductor devices can fail when moisture ingresses into and/or is generated within the plastics material. This can occur when the device becomes hot during, for example, surface mounting on a circuit board and/or during operation of the device. Its occurrence can be investigated by the device manufacturer in a reliability test, when the manufactured devices are pre-conditioned in conditions of humidity and heat. It is found that the moisture can attack and corrode the aluminium bond pads and the aluminium connection wires. Severe inter-granular corrosion of the aluminium material can occur, leading to an increase in electrical resistance and even to ultimate failure in electrical contact.
It is known to form connection wires of gold, instead of aluminium. Gold itself is resistant to corrosion by moisture, so ensuring the integrity of the connection wires themselves. However, the use of gold wire is much more expensive than aluminium wire, and it is not as compatible with an aluminium bond pad. Thus, particularly in power devices and/or at high temperature operation, intermetallic Au-Al compounds can be formed that are mechanically brittle and electrically resistive. Furthermore, in the presence of moisture, the two metals gold and aluminium can act as a galvanic battery, encouraging significant electrolytic corrosion of the aluminium.
U.S. Pat. No. 4,768,081 describes devices having connection wires of gold, wherein some form of getter is provided to capture the moisture. The preferred getter in U.S. Pat. No. 4,768,081 is a barium-aluminium alloy (such as BaA14) that is dispersed as a fine-grained powder in a gas-permeable inert silicone rubber within the encapsulation. The whole contents of U.S. Pat. No. 4,768,081 are hereby incorporated herein as reference material.
In order to reduce the electrolytic corrosion of aluminium bond pads in devices with gold connection wires, Japanese patent application kokai JP-A-60-150657 and its English-language abstract in the Patent Abstracts of Japan (Vol.9 No.318) proposes screening the (small-gauge) gold wire with an evaporated aluminium coating. The aluminium coating provides a sacrificial material that dissolves in the moisture as aluminium ions, so that any such moisture that reaches the aluminium bond pad is already saturated with aluminium and so should not corrode the bond pad. However, where the gold wire is ball-bonded to the aluminium pad, the gold ball is not protected by the aluminium coating and is exposed to the moisture. Thus, the possibility of galvanic action remains. Furthermore, the process is expensive, not only in using gold wire, but also in coating it with aluminium, and the coating will be alloyed and destroyed in the area of ball bond formation. Difficulty is also foreseen in adapting the wire-bonding equipment to handle the aluminium-coated small-gauge gold wire, without damage of the aluminium coating. The whole contents of JP-A-60-150657 and its English-language abstract are hereby incorporated herein as reference material.
Corrosion-resistant aluminium wire is commercially available, for example, under the trade name Tanaka. Such wire is of aluminium doped (i.e. at a parts per million level) with another element such as nickel that is effective in preventing intergranular corrosion of the aluminium by moisture. The use of such a corrosion-resistant aluminium alloy (instead of pure aluminium) avoids the incompatibility problems of gold wire. However corrosion by moisture is still found to occur at, for example, the bond pads.
It is an aim of the present invention to combat moisture corrosion in a plastics-encapsulated semiconductor device that has an aluminium-based metallurgy for its connection wires and bond pads.
According to the present invention, there is provided a plastics-encapsulated semiconductor device having a sacrificial, additional wire of substantially pure aluminium that is less resistant to corrosion by moisture within the encapsulation than are aluminium-alloy connection wires.
Devices in accordance with the present invention may have the features set out in claim 1.
A corrosion-resistant aluminium material, such as nickel-doped aluminium, is used for the connection wires, whereas the sacrificial additional wire is of relatively pure aluminium material that serves as a corrodible getter of the moisture within the encapsulation. This moisture getter of sacrificial aluminium wire protects the bond pads, which may be of an aluminium alloy or of relatively pure aluminium, from severe moisture corrosion. Thus, the present invention provides a low-cost aluminium-based compatible metallurgical scheme for the bond pads and wires. The connection wires and sacrificial additional wire can be bonded using the same equipment.
The sacrificial additional wire may take a variety of forms, for example, as a parallel connection, or as a so-called xe2x80x98stitch-bondxe2x80x99, a wire loop, or a bonded ball stump.
More than one such sacrificial additional wire in accordance with the invention may be provided within the encapsulation, for example at different locations within the encapsulation as and where space permits.
A sacrificial additional wire in accordance with the invention may be bonded in parallel with a connection wire, between the same bond pad and the same electrical terminal area as the connection wire. Where insufficient space is available on and between the bond pads and/or terminal areas, a sacrificial additional wire in accordance with the invention may be severed near its bond to form a stump, or it may be bonded in an electrically isolated loop.
Particularly in a power device, the bond pad of a main current path through the device body may have more than sufficient space available for the additional bonding thereto of one or more such sacrificial additional wires in accordance with the invention.
Where there is sufficient space available, the sacrificial additional wire may be bonded on an electrically conductive mount which carries the device body or on a further bond pad (even, for example, an electrically inactive bond pad) that may be provided on the device body. However, preference should be given to bonding the sacrificial wire to active bond pads of the device body, in order to maximize the protection for these bond pads.